Using Circuit Making Software for DFM: How to Prevent Tombstoning

Halloween, it’s that time of year again. Soon the streets will be filled with children dressed up in their scariest costumes ringing your doorbell and yelling “Trick or treat!” Already there are spooky decorations everywhere, and graveyard scenes seem to be one of the most popular themes.

There aren’t many things in life more frightening than the idea of weird and unnatural occurrences in a graveyard. Did you know though that a weird graveyard-like thing can happen right on the printed board that you’ve just designed? I’m talking about tombstoning. During the reflow solder process, your smaller passive chip components may lift up on one side and stand straight up like a tombstone. If you haven’t heard of this before, take a look at this short video. Doesn’t that just scare the socks off of you? Call Scooby and the gang, we’ve got a mystery to solve!

As you can see from the video, tombstoning is a very real manufacturing problem that can be prevented with tighter design standards and more accurate manufacturing processes. Let’s take a look at some of the causes of tombstoning and how you can help to prevent it.

Manufacturing issues that can cause tombstoning

The root cause of tombstoning, which is also known as the “Manhattan effect” or the “drawbridge effect”, can be traced to uneven wetting forces on the component during solder reflow. If the solder on one end of the component melts before the other end, it will cause an imbalance in the soldering process. This imbalance between the two terminal ends of the component can pull the component towards the wet or melted side, and cause the tombstoning effect.

There are many factors that can contribute to this solder imbalance. One of those is an incorrectly placed component that is not aligned on its pads. You can see how the misaligned component pulls towards the pad that it has the most contact with.

In addition to incorrectly placed components, there are other factors that can contribute to the tombstoning effect:

These are problems that can only be resolved by improving the accuracy of the manufacturing processes. If your manufacturer is unable to resolve these kinds of problems, it may be time to look elsewhere. There are other contributing factors that you can address on the design side though that will help to mitigate the tombstoning problem.

Passive chip components soldered on a PCB

How to prevent tombstoning using making software

One area of design where you can really help to reduce the chance of tombstoning when working with making software is in the component pad patterns. If the pads that you are using are not the correct size for the component, or if they are spaced too far apart, it can affect the thermal mass of the solder joints. If a solder joint has less mass than its solder will reflow sooner. The wet solder will then exert a pulling force on the component that may overwhelm the other end and cause the component to tombstone. You can avoid these problems by following the manufacturer’s DFM rules when creating your pad shapes.

The orientation of components on the board can also contribute to solder reflow problems. A component that is oriented on the board so that it travels parallel through the oven’s conveyor will have one terminal end heated before the other. This can also lead to tombstoning. Passive chip components should be oriented so that both ends are heated at the same time whenever possible.

Another area where you can help reduce the tombstoning problem is in making sure that the two pads of the component are the same size. Different sized pads on the same component will have different thermal masses, which will cause one pad to reflow before the other one resulting in tombstoning. This shouldn’t be a problem though since it is typical to use the same pad shape for both pins of the component.

Inspecting the quality of soldered components on a PCB

Managing the conflict between design and manufacturing

As we discussed earlier, the pad shapes for the two pins of a passive component should be the same size. However, this best practice for manufacturing can conflict with the routing requirements. Electrical considerations often dictate that one side of the component should have a very wide trace to connect it to power or ground and that connection is usually made to a plane directly through a via. Meanwhile, the other side of the component is usually connected through a thin trace.

Although the pad shapes are the same size, the different sized routing may cause the thermal mass of the pads to be different from each other. The wider traces, via, and ground plane will act as a heat sink for the pad that they are connected to. This will cause the pad to take more time to heat up than the pad that has the thinner traces on it. The result of all of this is a reflow timing difference between the two pads that can cause the component to tombstone.

This situation is the classic dilemma of the ; trying to satisfy the needs of both electrical engineering and manufacturing. In this case there isn’t a “right way” or a “wrong way” to resolve the problem. The best thing that you can do is to get both electrical engineering and manufacturing to agree on a strategy that will satisfy all needs of the design.

Plastic decorations of graveyard tombstones shouldn’t fill us with fear, but we should have a lot of respect for tombstoning during manufacturing. Now that you have a better understanding of what the problem is, you will be better prepared for it. With good design practices and open communication with your manufacturer, you will be able to bury this problem for good.